In ATM distribution systems, the physical layer is defined as a functional group comprised of hardware, software and transmission media which converts an ATM cell stream into bits to be transported over the transmission media. The physical layer supports the transmission and reception of these bits. Examples of transmission media are optical fiber, coaxial cable, free space, and twisted copper pairs.
In many cases the network is configured in a point-to-multipoint or bus configuration on a shared media, where several devices receive from and transmit to a single station. Examples of a shared media include free space propagation such as in radio systems, where a base station communicates with a number of remote stations, or a computing network in a small business in which all of the terminals are connected to a central server by a single pair of wires running throughout the building.
ATM networks transfer information in the form of cells. These cells contain both a data field and a header field. The data field contains user data, while the header field contains routing/identification and error control fields. Packet based networks transmit information in the form of packets which have similar structures to the cells in ATM networks. When used herein, the term cell refers to any type of cell or packet in a communications network.
In cell based networks, devices are granted permission to transmit one or more cells from a buffer which contains cells awaiting transmission. The permission can be in the form of a grant, which allows the device to transmit a cell in an upcoming opportunity. The term cell opportunity, when used herein, refers to an allocation of time or frequency which can be used to transmit a cell. For networks which are based on a form of time division multiplexing or time division multiple access, a grant can be received in a frame which indicates that a cell opportunity in a subsequent frame can be used.
In an ATM network, connections are established between devices on that network and are termed Virtual Circuits (VCs). The virtual circuits have parameters associated with them which indicate the data rate the virtual circuit provides and the type of service. The type of services which can be provided include Constant Bit Rate (CBR), Variable Bit Rate (VBR) or Available Bit Rate (ABR) transport on a virtual circuit. These virtual circuits can exist between devices on the shared media and a central receiving point, as well as on point-to-point connections.
When a virtual circuit is established between two points in an ATM network, there are characteristics which are associated with that virtual circuit. One characteristic is the data rate for the virtual circuit. The data rate for the virtual circuit will establish how frequently cells are transmitted.
Simultaneously with the development of ATM technology, there have been advances in Fiber-to-the-Curb (FTTC) technology in which devices are connected to the telephone central office via a network of optical fibers connecting the central office to single network points called Optical Network Units (ONUs) which in turn connect to the subscriber residence via a coaxial cable, and to the devices in the residence via a passive splitter and in-home coaxial wiring. In these FTTC networks, signals can be routed to the residence via a single coaxial cable connecting the residence to the ONU, but the passive network in the home results in a shared media, with all of the devices in the residence needing to transmit over a single coaxial cable.
One of the goals of the present invention is to provide one or more embodiments which permit the transport of ATM cells over a point-to-point and point-to-multipoint networks in a manner such that devices on that network are issued grants to transmit cells to the ONU, and that these grants are allocated in a manner that guarantees that the virtual circuits that have been established between the central station and the devices have the Constant Bit Rate (CBR), Variable Bit Rate (VBR) or Available Bit Rate (ABR) services that they have contracted for in the system, according to the parameters specified in the Quality of Service (QoS) associated with each of the virtual circuits. The QoS defines the basic parameters of the VC including the cell loss rate and average delay.
Another goal of the invention is to provide a method for issuing grants such that cells associated with CBR services have a minimum variation between cells, resulting in a low Cell Delay Variation (CDV).
These and other features and objects of the invention will be more fully understood from the following detailed description of the preferred embodiments which should be read in light of the accompanying drawings.